docs: def derive_and_set_slug(self, set_name_sort=True, set_slug=True):
          """
          Derive `slug` field from `title` unless it is set in its own right.
  
          This method is called during `save()`
          """
          # `title` is the primary required name field. It must be set.
          if is_empty(self.title):
              raise ValueError(
                  u"%s.title cannot be empty at save" % type(self).__name__)
          # if empty, `slug` is set to slugified `title`
          if set_slug and is_empty(self.slug):
              self.slug = slugify(self.title)